1. Field of the Invention
The present invention relates broadly to the field of nondestructive examination and more specifically to the nondestructive examination of wiring. Even more specifically, the present invention relates to the nondestructive examination of wire insulation and coatings.
2. Description of the Related Art
Electrical wiring is critical to the operation of most modern day equipment and, in its operation, is subjected to heat, cold, moisture, vibrations, tension and other environmental conditions which eventually may cause the wire insulation and even the wire conductor to fail. In most cases, these environmental and operational conditions are modest and wiring is used for years, but in some cases these conditions are extreme and cause the insulation to become brittle and crack. The cracks expose the underlying wire conductor and become a potential source for short circuits and fire.
There are few available methods to evaluate the condition of the insulation on electrical wiring. Typical wire inspections are done visually and often after the fact, in response to an instrument or system failure. A visual inspection often fails to detect many cracks and flaws because the cracks and flaws are not visible or are located in spaces that are difficult to see. Furthermore, a visual inspection offers little quantitative information about the condition of the wire insulation. Some techniques require a section of wire to be removed for laboratory testing. These techniques are undesirable due to their destructive nature. There are also techniques that involve application of voltage to the wire to detect current leakage. The current leakage is indicative of an insulation failure, such as cracking, but does not provide predictive information on the state of the insulation. Some of the voltage application techniques are conducted in air, while others imbed the wires in a conductive medium. Additionally, some involve high voltage while others have been designed to detect leakage at low voltages.
Meeker, T. R., and Meitzler, A. H., xe2x80x9cGuided Wave Propagation in Elongated Cylinders and Plates,xe2x80x9d Physical Acousticsxe2x80x94Principles and Methods, edited by W. P. Mason, Academic Press, N.Y., Vol. 1, Part A., 1964, pp.111-167; Thurston, R. N., J. Acoust. Soc. Am., 64, 1, 1-37, (1978); McNiven, H. D., Sackman, J. L., and Shah, A. H., J. Acoust. Soc. Am., 35, 10,1602-1609, (1963), and Abramson, H. N., J. Acoust. Soc. Am., 29, 1, 42-46, (1957) examined acoustic guided wave propagation in cylindrical geometry. Madaras, E. I., and Anastasi, R. F., xe2x80x9cPseudo-Random Modulation of a Laser Diode for Generation Ultrasonic Longitudinal Waves,xe2x80x9d 26 Annual Review of Progress in Qualitative Nondestructive Evaluation, Montreal, Quebec, Canada, July 1999, and Anastasi, R. F. and Madaras, E. I., xe2x80x9cPulse Compression Techniques for Laser Generated Ultrasound,xe2x80x9d IEEE International Ultrasonics Symposium-1999, edited by S. C. Schneider and B. R. McAvoy, IEEE Ultrasonics, Ferroelectronics, and Frequency Control Society, 1999, both incorporated herein by reference, examined ultrasonic guided waves for characterization of wire.
There are numerous methods for wire nondestructive examination that involve investigation of the conductor. One method is Time Domain Reflectometry (TDR) and another is Standing Wave Reflectometry (SWR). These methods and related variants are sensitive to the conductor but are only mildly affected by the condition of the insulation. Furthermore, these methods only detect insulation failure.
U.S. Pat. No. 4,380,931 (Frost, et al.), utilizing a plurality of noncontacting ultrasonic transducers in cooperation with a magnetic field, is applicable only to conductive wires, and more specifically only to solid cylindrically shaped objects, not stranded wires with insulation. Furthermore, only torsional waves are produced in a solid conductor. U.S. Pat. No. 5,457,994 (Kwun et al.) utilizes the magnetoresistive effect to generate and detect acoustic waves to measure the condition of conducting wires, but does not detect the surrounding materials"" condition. U.S. Pat. No. 4,593,244 (Summers et al.) is limited to measuring the thickness of conductive coatings that are on ferromagnetic substrates. In general, electrical wires that are usually of interest do not utilize a conductive coating and, in addition, the thickness of a wire coating is, in general, not the only concern that faces most electrical wire users.
U.S. Pat. Nos. 4,659,991 (Weischedel), 4,929,897 (Van Der Walt), 4,979,125 (Kwun et al.), and 5,456,113 (Kwun et al.) teach methods that are applicable only to ferromagnetic materials. None of the aforementioned patents teach non-destructive examination of wire insulation. U.S. Pat. No. 4,659,991 (Weischedel), detects shape changes in a cable and uses magnetic fields to sense the shape changes, but is not relevant to wire insulation. U.S. Pat. No. 4,929,897 (Van Der Walt), also detects shape changes in a cable and also uses magnetic fields from a different sensor geometry than Weishedel to sense the shape changes, and again is not relevant to wire insulation. U.S. Pat. No. 4,979,125 (Kwun et al.) tests a cable, rope or metal strand (which are not insulated) by first striking the cable with an impulse such as a hammer or electromagnetically driven plunger, and then detecting the resulting vibrations with a magnetic sensor. U.S. Pat. No. 5,456,113 (Kwun et al.) tests ferromagnetic cables and ropes (which are not insulated) by inducing and detecting acoustic/ultrasonic waves by a magnetorestrictive means.
It is therefore an object of the present invention to provide a nondestructive method and apparatus for evaluating the condition, both prior to and subsequent to failure, of the insulation on electrical wiring.
It is another object of the present invention to provide a nondestructive method and apparatus for evaluating the condition of wire insulation quantitatively, giving the user information on the expected safe remaining life of the wire.
It is another object of the present invention to provide a nondestructive method and apparatus for evaluating the condition of either ferromagnetic or nonferromagnetic insulation on electrical wiring.
It is yet another object of the present invention to provide a nondestructive method and apparatus to utilize ultrasonic wave generation to evaluate the condition of electrical wire insulation.
It is yet another object of the present invention to provide a nondestructive method and apparatus for evaluating the condition of electrical wire conductors.
It is yet another object of the present invention to provide a nondestructive method and apparatus for evaluating the condition of wire coatings.
Still other objects and advantages of the present invention will in part be obvious and will in part be apparent from the specification.
The present invention uses the generation and detection of acoustic guided waves to evaluate the condition of the insulation on electrical wiring. Low order axisymmetric and flexural acoustic modes are generated in the insulated wire. These modes travel partially in the center conductor and partially in the outer insulation. The stiffness of the insulation and the insulation""s condition affect the overall wave speed and amplitude of the guided wave. Thus, the measurement of wave speed will in part be a measurement of material stiffness and, in part, be a measurement of insulation condition. Analysis of the received signal provides information about the age or useful life of the wire insulation.
Although there are other, higher order modes that are generated, the two lowest order modes mentioned are generally the easiest to excite. The flexural mode is one of the largest generated. Although the axisymmetric mode is generally small, it is easy to measure, and thus desirable to use. Little or no axisymmetric mode is generated with the laser generation method, to be discussed later, most likely reflecting the small area of generation in contrast to the larger area of a transducer. The particular mode to be utilized is determined based on the ease of generation, low attenuation, and sensitivity to the damage being tested for. Some testing of a baseline sample will generally be needed to determine which mode to utilize.
The wave speed and attenuation of the waves are measured and provide information about the physical condition of the insulation. The speed measurement is related to the stiffness and density of the material components. The attenuation measurement is related to the structure and microstructure of the component materials, such as microcracks in the insulation. In general, wire insulations are of a polymer base and have much lower stiffness characteristics than the center conductor, which is usually copper or aluminum. Because copper and aluminum have a much higher wave speed than polymers, the effect of wrapping insulation on a cylindrical shaped conductor will be to lower the wave speed of the guided wave. As the insulation is aged, it will loose its plasticity and harden, which will lead to cracks, exposing the center conductor, which could lead to electrical shorting. As the insulation hardens, the coating material will stiffen, which will cause the wave speed to be greater. The frequency content and amplitude information provide an indication of the insulation""s condition, such as chaffing, cuts, nicks, cracks and flaws. Each of these conditions will attenuate the signal. Both flaws and degradation will affect the signals. For example, a nick in the insulation changes the frequency content of the signal, whereas degradation alters the signal speed and attenuation. The present invention is applicable to any conductor material, with the details of the wave motion depending on the relative constituents.
In accordance with the present invention, signal transmission occurs at one location on the electrical wire to be evaluated, and detection occurs at one or more locations along the electrical wire. The number and position of detection locations depends on the user""s preference. In one embodiment, transmission and detection occurs at one location, which is especially effective for evaluating the termination points of wire, such as at connectors, as well as for detecting signals reflected from flaws. For connectors, one transducer can be used to transmit the signal to the connector and detect the reflected signal. The transducer would be positioned as close as possible to the connector. Evaluation can consist of viewing the waves or estimating the wave velocity based on the distance of the transducer from the connector. With a flaw, the existence of the flaw would produce a signal anomaly.
In another embodiment, detection occurs at one or more locations separate from the transmitting location. This configuration generally has good signal to noise. The positioning of the transducers is dependent upon the anticipated region of criticality. Often certain areas are more suspect than others and should be inspected with more detail and frequency. General areas could be spot checked if desired. Two simultaneous measurements can be taken to generate both attenuation and speed values. If the distances between any two pairs of transmit or detect transducers are not equal, then the difference between the time of the received signals divided into the differences in transducer spacing will give the velocity of the ultrasonic wave. Additional receivers can be used to improve measurement accuracy.
In further alternative embodiments, either the transmitter transducer or one or more receiver transducers may be angled at other than 90 degrees to the wire.
Generation of the guided waves can be accomplished by imparting a pressure pulse on the wire. Alternative embodiments include generation via a laser, such as a Q-switched laser or a laser diode.
The detected signal can be further processed to extract the material properties of interest with respect to the wire insulation. One method of processing is to apply the generation and detection to wires exhibiting a range of conditions, both acceptable and unacceptable, to produce a look-up table of velocity or attenuation properties for that specific wire type that could then classify an unknown wire specimen. Another method is to apply modeling. By setting up the differential equations for the particle motions and stresses and strains, and matching the boundary conditions at the interface of the conductor to insulation and the insulation to air, an ultrasonic propagation model for a wire covered by insulation can be developed. A further general description of such modeling can be found in the earlier references to Meeker, T. R., and Meitzler, A. H.,; Thurston, R. N.,; McNiven, H. D., Sackman, J. L., and Shah, A. H.,; and Abramson, H. N. Then, using known properties for the conductor and the dimensions of the wire, the properties of the insulation can be inferred utilizing mathematical techniques via a computer. Examples of commercially available software include Disperse, as well as general software in commercial packages such as Numerical Methods in C, Mathematica, MatLab, and IDL. In a similar method, finite element method or finite difference modeling software can be used to accomplish the same result, although generally more expensive.
The present invention provides the capability to measure velocities, frequencies, and magnitudes. The system is adapted to measure characteristics that are relevant to the flaw/degradation being tested for. For example, the signal""s frequency content would be significantly changed and the attenuation would be worse for severe chafing.
In addition to the evaluation of insulation, the present invention can also be used for evaluating coatings, as well as the conductor itself. It can also be used to evaluate stranded wire. The system is adapted, frequency for example, to measure the particular constituent condition. The invention can be used for any layered media, including cylindrical or rectangularly shaped structures, and including media that is not conductive. The stiffness of various layers would determine the ultimate efficacy of any testing. At the lowest frequencies, it would test the whole structure, but at higher frequencies, it would tend to test the layers with the lowest stiffness.